1. Field of the Invention
This invention pertains to a novel process for stabilizing polyethylene resins having pendant vinyl and/or vinylidene groups against changes in viscosity under melt process conditions. The process comprises adding a viscosity-stabilizing amount of transition metal or transition metal salt to the polyethylene resin prior to or during melt processing operations. The resulting polymer composition is a new melt-stabilized, crosslink-resistant, substantially gel-free polymer composition having many uses. The novel compositions are prepared free of metal deactivators.
2. Technical Background
Polyethylenes are a known class of thermoplastic polymers having many members. They are prepared by homopolymerizing ethylene or interpolymerizing (e.g., copolymerizing) ethylene with one or more alpha-olefins having from 3 to about 18 carbon atoms by known polymerization reactions and conditions. The terms "polyethylenes" and "ethylene polymers" will be used interchangeably herein to refer to such homopolymers and interpolymers (e.g., copolymers, terpolymers) of ethylene. The viscosity of polyethylenes which have pendant vinyl and/or vinylidene groups tends to change during melt process operations. E.g., during extrusion, molding, etc. Such thermally-induced changes in viscosity have been attributed to the changes in molecular weight and/or linearity of the polymers caused by crosslinking.
A wide variety of "stabilizers" have been developed to reduce the changes (e.g., crosslinking) that can occur during melt processing or under conditions of use. Many of the stabilizers are organic compounds which are classified in the plastics industry as antioxidants. Many antioxidants tend to function as free radical scavengers and they interact with free radicals that are formed during polymerization or in the presence of air or other oxidizing medium. Antioxidants are a known class of stabilizers which includes, for example, hindered phenols, triaryl phosphites, aromatic amines, hydroxylamines, and the like. Antioxidants have been added as stabilizers to polyethylenes with mixed success. Typically, such antioxidants have protected polyethylene articles against oxidative degradation at ambient conditions but have not been particularly effective at protecting the polymer against thermally-induced changes in viscosity during melt processing.
The text "Plastic Additives Handbook", Edited by R. Gachter and Muller and distributed in the United States of America by Macmillan Publishing Co., New York, NY (1985) describes Antioxidants in Chapter 1 and the mechanisms by which such compounds are thought to work. It describes polymers that change properties under melt process conditions due to chain scission (e.g., polypropylene) and polymers that change properties due to crosslinking (e.g., low density polyethylene (LDPE)) and suggests that processing stabilizer systems commonly used in polypropylene (i.e., phosphites and a long-term heat stabilizer in overall concentrations up to 0.1%) could be used as process stabilizers for linear low density polyethylene (LLDPE). In Chapter 2, the text confirms the literature in describing the thermo-oxidation of polyolefins as proceeding by a free radical chain mechanism in which hydroperoxides are key intermediates.
Hydroperoxides undergo thermally induced (120.degree. C. and higher) homolytic decomposition to free radicals, which in turn initiate new oxidation chains which attack the polymer and cause degradation. This homolytic decomposition reaction is said to be catalyzed in a redox reaction by the presence of catalytic amounts of certain metal ions, particularly transition metal ions, such as iron, cobalt, manganese, copper and vanadium. The author then states that the presence of such metal ions in the autooxidation of a hydrocarbon increases the decomposition rate of hydroperoxides and the oxidation rate to such an extent that even in the presence of antioxidants, the induction period of oxygen uptake is drastically shortened or completely eliminated. Even at rather high concentrations, hindered phenols or aromatic amines reportedly do not retard the oxidation rate sufficiently. A more efficient inhibition is allegedly achieved by using metal deactivators (e.g., copper inhibitors).
A variety of metal deactivators are described in Chapter 2 in the Gachter et al. handbook and a method of testing is set forth on page 82. In the test, the polyolefin resin and stabilizer are homogenized (i.e., thoroughly blended) in a suitable lab scale kneader (Brabender plastograph), or by milling and adding in the end 1% of a fine copper powder or 0.1% copper stearate, making a compression molded plaque, and then oven aging the plagues to determine polymer changes over time. Test results are commercially important because of the wide use of polyolefin insulation over copper conductors. In such applications the author states that it is mandatory to combine a metal deactivator with an antioxidant if the metal deactivator does not contain moieties with radical scavenging function. Information is presented in Table 1 on page 84 showing combinations of metal deactivators and phenolic type antioxidants used to protect polyethylene in contact with copper. The need for metal deactivators is emphasized by the teaching in "Additives for Plastics" by J. Stepak and H. Daoust, Springer-Verlag New York Inc. (1983) at pages 182-183 that heavy metal ions (Co, Cu, Mn, Fe, Pb) which catalyze the hydroperoxide decomposition are present in polymers from contact with metallic parts of reactors and processing machines.
Attempts have been made to counteract the metal/metal ion catalyzed peroxide decomposition reaction in polyolefins by including a material in the polymer which reacts preferentially with the peroxide or its decomposition products. Such materials being referred to in this patent application as "sacrificial reducing agents". For example, Black (U.S. Pat. No. 4,122,033) allegedly stabilized organic materials against autooxidation by including at least 100 parts per million (ppm) of a transition metal containing compound and certain (1) aliphatic amines, (2) alkyl selenides, or (3) alkyl phosphines or phosphites.
Similarly, Chiquet (U.S. Pat. No. 4,931,488) included starch in a thermoplastic polymer (e.g., polyethylene) to make a thermoplastic composition which allegedly degrades under the action of heat, ultraviolet light, sunlight and/or composting conditions. Chicquet used iron and another transition metal compound (e.g., copper stearate) to catalyze the degradation of the starch.
In view of these representative teachings about the catalytic effect which transition metals and metal ions have on the thermooxidation of polyolefins (e.g., polyethylenes) and how this catalytic effect is controlled by adding materials which react preferentially with peroxides to "protect" the polymer, it was a surprise to learn that small amounts of a transition metal can thermally stabilize certain polyethylenes under melt process conditions without the "benefit" of metal deactivators.